The present invention relates to a heat transfer film, in particular, a protective layer transfer film in which a protective layer is transferred thermally, and, more specifically, to a protective layer transfer film capable of imparting durability such as resistance to light, weather, abrasion, chemicals and solvent to an image produced by means of heat transfer printing or the like.
Heretofore, a variety of heat transfer printing methods have been widely used as simple methods of printing. These heat transfer printing methods can easily produce images of various kinds, so that they have been conveniently utilized to make those printed matters whose number of printing is relatively small, such as ID cards.
Further, when a colored image is desirable as in the case of a photograph of face, there has been employed a heat transfer printing method using a heat transfer printing film which is obtained by sequentially providing areas of heat transfer coloring layers of yellow, magenta, cyan, and, if necessary, black on a continuous base film, a set of these sequential areas being repeatedly provided.
Heat transfer printing films of the above type can be broadly divided into two types: a heat transfer printing film of the so-called hot-melt transfer type whose heat transfer printing layer is softened by the image-wise application of heat and transferred to an image-receiving material, whereby an image is recorded on the image-receiving material; and a heat transfer printing film of the so-called sublimation transfer type in which a dye contained in its heat transfer printing layer is sublimed (migrated) by the application of heat and only the dye is transferred to an image-receiving material, whereby an image is formed on the image-receiving material.
However, it is not suitable to make ID cards or the like by the use of the above heat transfer printing films. For example, a heat transfer printing film of the hot-melt transfer type has such a shortcoming in that although it can readily produce images of letters, numbers or the like, the images produced are poor in durability, especially in abrasion resistance.
On the other hand, a heat transfer printing film of the sublimation transfer type has a shortcoming in that although it can precisely produce an image having gradation such as a photograph of face, the image produced is poor in durability such as resistance to light, weather and abrasion because the image contains no vehicle unlike an image printed by using an ordinary printing ink.
For example, a method in which an ultraviolet-absorbing agent or an antioxidant is incorporated has been known as a method for overcoming the above-described shortcomings. The light resistance and the like of the printed images can be improved to a certain extent even by such a method. However, a method in which an ultraviolet-absorbing agent is simply incorporated into a resin used for forming a protective layer has such a problem in that the effect of the ultraviolet-absorbing agent is decreased with time because the agent is volatilized or decomposed by heat. Japanese Laid-Open Patent Publication No. 212974/1993 discloses, as a method for solving this problem, a method in which a reactive ultraviolet-absorbing agent is reacted and combined with a resin used for forming the dye-receiving layer of a heat transfer image-receiving sheet. However, a dye which migrates when heat is applied and produces an image in the sublimation transfer printing process exists relatively near the surface of the dye-receiving layer, so that it is not effective to entirely impart ultraviolet-absorptivity to the dye-receiving layer. Moreover, there has been a problem in that when the amount of the ultraviolet-absorbing agent is increased, the properties inherent in the resin of the dye-receiving layer are impaired, and other properties are also marred, for instance, the image produced is spread.